Fatigue failure is one of the major failure modes of mechanical parts and engineering components. It is mostly in a form of brittle failure and liable to causing heavy economic and property loss. Therefore, for a long time, the problem of fatigue failure has aroused wide concerns. Through enormous experimental research and practical engineering application, some description methods of fatigue failure have been formed based on different concepts. Among them, the fatigue crack growth rate description method formed based on the concept of damage tolerance has achieved a good effect in the fatigue life evaluation of cracking members. Fatigue crack growth rate is not only an important parameter representing resistance of a material to fatigue crack growth but also an important extension of the design concept of fatigue strength and total life based on elastic-plastic mechanics. With the constant development and perfection of fracture mechanics, this method is gaining more extensive application in fatigue design and analysis. For mechanical parts and engineering components, due to complex structure and working conditions, fatigue failure is mostly in a mixed mode, so the research on failure resulting from mixed-mode fatigue crack growth has very important theoretical significance and engineering value.
In view of fatigue crack and stress, fatigue cracks may be classified into mode I (the crack endures the action of vertical normal stress), mode II (the crack endures the action of in-plane shear stress) and mode III (the crack endures the action of out-of-plane shear stress). At present, many experiment researches have been done on fatigue crack growth rate, but they mostly use standard fixtures and specimens to test the growth rate under the action of mode-I fatigue load, while little is researched on mode II, mode III and mixed-mode fatigue crack growth rates and little experimental data is available. Today, commercial fatigue testing machines are dominated by axial tension-compression testing machines. Even if an axial tension-compression+torsion testing machine is adopted, standard fixtures and specimens can be used in mode I, mode III, and mode I-III mixed-mode fatigue crack growth tests only. Knowing the growth rates of cracks under the actions of various types of fatigue loads will help realize precise prediction on fatigue life of mechanical parts and engineering components in a complex stress field.